The present invention is based upon the phenomenon that oxygen has a quenching effect on the molecular luminescence of various chemical compounds and that this effect can be employed for imaging oxygen concentrations of the body portions of animals and humans. Animals, especially mammals, are dependent upon having adequate oxygen supplies in their body tissues. In mammals, the circulatory system employs specialized oxygen-carrying molecules in the blood to deliver oxygen from the lungs to other tissues throughout the body. Thus, every organ in the body contains oxygen in varying amounts and concentrations in every tissue. The distribution of oxygen in tissue can be indicative of structure, anomalies, defects or disease. U.S. Pat. No. 4,947,850 provides a detailed discussion of such technologies.
Co-pending U.S. Ser. No. 08/022,190 entitled Method and Apparatus for Oxygen Mapping discloses improved methods and apparatus for imaging internal body structures of animals. The apparatus and methods disclosed in this application are directed to measuring tissue oxygenation through the skin using oxygen dependent quenching of phosphorescence. In addition, there have been additional patents directed to this technology.
U.K. patent application No. GB 2,132,348A, published Jul. 4, 1984, disclose the use of fluorescent materials to measure levels of oxygen in blood both in vitro and in vivo using a fiber optic probe or catheter.
The prior art has disclosed indwelling devices for use during measurement of various blood parameters. For example, U.S. Pat. No. 3,787,119 discloses a catheter having a microlamp and a photosensitive element and other elements including a cup-like element for use in receiving blood and providing electrical output signals by means of wires extending through the catheter.
U.S. Pat. No. 3,814,081 discloses an optical measuring catheter employing fiber optic means for use in measuring oxygen saturation in blood, as well as blood pressure.
U.S. Pat. No. 4,200,110 discloses a fiber optic pH probe which includes an ion permeable membrane which encloses a guide containing solid material comprised of a hydrophilic copolymer having a pH sensitive dye attached thereto. The probe functions by optically detecting a change in color of the pH sensitive dye when excited by light. A phenol red dye is employed so that it absorbs light at a particular wavelength, with the amount of light being absorbed varying in dependence upon the pH level.
U.S. Pat. No. 4,476,870 discloses a fiberoptic oxygen partial pressure probe. This probe includes a hydrophobic gas permeable envelope which contains an adsorptive support which contains a fluorescent dye. Use of the probe for measuring partial pressure of gaseous oxygen in the bloodstream is based on the principle of dye fluorescent oxygen quenching. With the probe in place within a bloodstream, fluorescent dye is excited by light having blue wavelength, thus causing the dye to fluoresce at a green wavelength with the intensity of emitted light decreasing (quenching) with increasing levels of the partial pressure of gaseous oxygen in the bloodstream.
U.S. Pat. No. 5,127,405 discloses a fiber optic probe incorporating a luminescent composition which is used to monitor conditions of a subject. A response light from the fiber optic probe is detected and a frequency domain presentation of the response light is derived. Characteristics of the frequency domain representation are used to derive values for luminescent lifetimes or similar decay parameters and these values in turn are translated into the values of the conditions to be sensed.
Finally, U.S. Pat. No. 4,898,175 discloses an apparatus in which an illuminating light is fed by a device emitted from the tip part of an insertable endoscope. The endoscope is inserted into a body cavity and is radiated on to a part of the body to be observed. This illuminating light, having passed through a living body tissue, is imaged by an imaging device provided outside the body. The imaging device delivers a picture image signal to a signal processing device. The signal processing device processes the signal and outputs a video signal to a display device. This device displays the image observed within the living body. See also U.S. Pat. No. 4,947,850.
In addition to the above technologies, oxygen electrodes have also been designed for transcutaneous oxygen measurements. Oxygen electrodes, in contrast to systems which are based on the oxygen dependent quenching of phosphorescence, utilize substantial amounts of oxygen. The oxygen permeability of the skin is low and oxygen consumption by the electrodes can seriously deplete the oxygen pressure at the surface of the skin, resulting in measured oxygen values which are artificially low and which are strongly dependent upon blood flow in the immediate vicinity of the electrodes. In general, an oxygen electrode system must compensate by heating the skin to well above normal values in order to maximally dilate the vessels. In the phosphorescence method, the negligible oxygen consumption by the measuring system will permit the use of only one modest heating, primarily to overcome possible vasoconstriction due to depressed body temperature to assure uniform conditions among subjects. Oxygen electrodes further require calibration before each use. The calibration cannot alter with the time of measurement.
It would be desirable to provide an improved device and method for measuring oxygen pressure which could be applied to the outside of the skin and which could be used to obtain rapid and accurate oxygen pressure measurements. These and other objects of the present invention will become clear from the summary and detailed description which follow.